This study assessed stability and lability of clinically relevant problem behaviors of children. The sample (n = 468) consisted of both boy and girl outpatients of a community child guidance clinic. Child Behavior Checklists of 6- to 11- and 12- to 16-year-olds were filled out by parents at intake and again 6 months and 18 months thereafter. Using a longitudinal analysis, the following derived from Achenbach's Child Behavior Checklist were examined: individual behavior problems, behavioral syndromes, and groupings of externalizing and internalizing behavior problems. Analyses showed that stability and lability of problem behaviors varied by age and sex. As a group, children showed decreases in the intensity of behavior problems, although maintaining stable rank ordering relative to peers.